1. Field of the Invention
The present invention is directed to an LED-based phosphor illuminator.
2. Description of the Related Art
Light emitting diodes (LEDs) are rapidly finding acceptance in many lighting applications. Compared with incandescent light bulbs, LEDs are more efficient, have longer lifetimes, and may be packaged in a wide variety of suitably shaped and sized packages.
In particular, so-called white-light LEDs are become more popular for illumination applications. In these white-light LEDs, the light-producing element is typically an LED that emits light at a relatively short wavelength, such as blue, violet, or UV. The light emitted from the so-called blue LED strikes a phosphor. The phosphor absorbs the blue light and emits light at one or more longer wavelengths, which may include discrete wavelengths in addition to continuous portions of the spectrum. The light emitted from the phosphor may be used to illuminate an object, or may be used for general lighting purposes.
Many of the characteristics that pertain to human vision, such as the (x,y) coordinates on the CIE color chart (or other suitable chart), and the so-called color temperature (which relates the emission spectrum of the phosphor to that of a blackbody having a particular temperature), are typically determined by the chemical properties of the phosphor itself, its interaction with the illuminating blue light, and the wavelength of the illuminating blue light.
There are additional factors that affect the performance of an LED-based illuminator, which are generally independent of the performance of the phosphor. For instance, a dominant contributor is typically the efficiency of getting phosphor-emitted/scattered light out of the device. A secondary contributor is typically the efficiency of the optical path between the blue LED and the phosphor helps determine the brightness of the illuminator. In other words, the higher the percentage of photons leaving the blue LED and striking the phosphor, the more output light emitted by the phosphor. In addition, many phosphors emit light in a Lambertian manner, with a similar or identical angular profile. For some applications, this Lambertian distribution may be too wide, and a narrower cone of light may be desired.
In general, the known optical systems fail to provide an LED-based illuminator that has both a high fixture efficiency (i.e., a high percentage of light leaving the blue LED that strikes the phosphor) and a relatively narrow beam angle (i.e., a relatively small angular distribution of exiting light, compared to a Lambertian distribution).
As specific examples, we consider three known references, and we note their deficiencies below.
As a first example, we consider United States Patent Application Publication No. US 2007/0267976 A1, titled “LED-based light bulb”, invented by Christopher L. Bohler, et al., and published on Nov. 22, 2007. FIG. 5 from Bohler is reproduced herein as FIG. 1 in the present application.
The lighting system 510 of Bohler includes a wavelength converting material such as organic or inorganic phosphor. The phosphor can be located in any suitable location, such as integrated into the LED 512, at a light guide 536, coated inside or outside the cover 522, contained within the cover 522, or a combination thereof. Examples of the organic transparent phosphors are the BASF Lumogen F dyes such as Lumogen F Yellow 083, Lumogen F Orange 240, Lumogen F Red 300, and Lumogen F Violet 570. Of course, it is also contemplated that other phosphors such as the rare earth complexes with organic component described in the U.S. Pat. No. 6,366,033; quantum dot phosphors described in the U.S. Pat. No. 6,207,229; nanophosphors described in the U.S. Pat. No. 6,048,616, or other suitable phosphors can be used.
The UV light rays 540 are emitted by the LEDs 512 and converted into white or visible light 542 by a phosphor 544. The phosphor 544 preferably includes two or more phosphors to convert the emitted light 540 to the visible light 542, although single component phosphors are embodied for saturated color light generation as well. The visible light 542 exits through the enclosure 522. In this embodiment, the phosphor mix 544 is disposed about or within a light guide 536 which is a planar panel disposed above the LED 512 such that the majority of the light rays 540 strike the panel.
Two issues are notable with the device 510 of Bohler.
First, a relatively small fraction of the light emitted from the LEDs 512 reaches the phosphor 544. The phosphor itself has a particular size and is located a particular distance away from the LEDs 512. Light emitted from the LEDs 512 has a particular angular distribution, typically a Lambertian distribution, such that a certain percentage of LED light strikes the phosphor 544, with the remaining light missing the phosphor and failing to generate any white light. This results in a reduced efficiency in the fraction of LED emission that is delivered to the phosphor, which may be significantly less than 100%.
Second, the light exiting the phosphor 544 leaves the phosphor plane and travels directly out to the viewer. In general, light emitted from a planar phosphor has a relatively wide angular distribution, which may be considered too wide for some applications. A more detailed explanation of this emission from a plane is provided in the following paragraph.
In general, light emitted from a phosphor is found to have a generally Lambertian distribution in power per angle. A Lambertian distribution has a peak that is oriented normal to the emitting surface (often denoted as 0 degrees), with an angular falloff of cos θ, where θ is with respect to the surface normal. This Lambertian distribution may be represented numerically by a full-width-at-half-maximum (FWHM) in angle, given by 2 cos−1 (0.5), or 120 degrees. For many applications, this FWHM of 120 degrees may be considered relatively wide. There may be instances when a more narrow or a more controllable beam is desired.
As a second example, we consider United States Patent Application Publication No. US 2008/0030993 A1, titled “High efficiency light source using solid-state emitter and down-conversion material”, invented by Nadarajah Narendran, et al., and published on Feb. 7, 2008. The '993 publication was originally published on Nov. 17, 2005 as PCT Application Publication No. WO2005/107420 with informal figures. FIG. 4 from Narendran is reproduced herein as FIG. 2 in the present application.
The embodiment in FIG. 2 may be used in interior spaces where general ambient lighting is required. As shown, the device includes phosphor plate 650 (for example YAG:Ce or other phosphors). The device also includes multiple semiconductor light emitting diodes 656 forming an array, such as LED/RCLED array 652. The array 652 is mounted on substrate 654 that may be of aluminum material. In an exemplary embodiment, substrate 654 may be circular. In the exemplary configuration illustrated in FIG. 2, the LEDs/RCLEDs are arranged in a spaced relation to each other and placed around the circular substrate.
In Narendran, the array of light emitting diodes are placed on the substrate so that the light emitting surfaces of the diodes face toward phosphor layer plate 650. In this manner, diodes 656 emit short wavelength light toward phosphor layer plate 650. As the short wavelength light impinges on the phosphor layer plate, four components of light results: reflected short wavelength light and down-converted light 660 and transmitted short wavelength light and transmitted down converted light 664. The short wavelength light and down converted light 660 is reflected, as shown, within the device to produce white light 662. The transmitted short wavelength light and down-converted light 664 is transmitted outside of the device to produce white light 66.
The device of Narendran has the same two issues as that of Bohler. First, the fraction of LED emission that is delivered to the phosphor may be significantly less than 100%. Second, the angular distribution of the white light may be especially wide, and even more so compared with the device of Bohler since there is both transmitted and reflected light propagating away from the phosphor toward the viewer.
As a third example, we consider U.S. Pat. No. 7,293,908 B2, titled “Side emitting illumination systems incorporating light emitting diodes”, issued on Nov. 13, 2007 to Karl W. Beeson, et al. FIG. 12 from Beeson is reproduced herein as FIG. 3 in the present application.
Light from an LED 702 travels without reflecting off any other optical elements to a wavelength conversion layer (phosphor) 902. A reflector 706 is adjacent to the wavelength conversion layer 902, on the side opposite the LED 702. Wavelength-converted light travels back toward the LED 702, with a lateral component determined by the emission angle distribution of the phosphor 902. The light then reflects off reflector 704, transmits through planar transparent element 802 and exits the device. The reflectors 704 and 706 are planar and parallel, and are longitudinally separated by separation distance 718.
The device of Beeson faces the same two issues as those discussed above for the previous two references. First, the fraction of light leaving the LED 702 that reaches the phosphor 902 may be significantly less than 100%, because of the nature of the free-space propagation between the LED 702 and the phosphor 902 (i.e., light rays may “leak out” of the propagation region and fail to strike the phosphor). Second, the wavelength-converted light that leaves the device has essentially the same angular distribution as the light emitted from the phosphor 902; the reflection off planar mirror 704 does not change the angular distribution of the light. This angular distribution may be too wide for some applications.
For these reasons and others, there exists a need for an LED-based illumination device that has a relatively high efficiency for light propagating from the LED to the phosphor, and has a light output angle distribution that is controllable and/or is narrower than that from the phosphor itself.